A powerful extremist Islamic group has advanced into Turkey's sphere of influence in northern Syria in the past five days, residents said on Tuesday, prompting Ankara to send military reinforcements to the area. Residents and sources in the opposition to President Bashar Al Assad said a Turkish armoured column crossed the border into Syria on Tuesday to check the advance by militiant organisation Hayat Tahrir Al Sham (HTS) on the city of Azaz in Aleppo governorate. The group — a coalition of smaller militant organisations, controls the northern rebel-held Idlib governorate. A key component of HTS, a group once known as Jabhat Al Nusra, was once Al Qaeda's Syrian affiliate and widely suspected of receiving covert support from Turkey. HTS, also known simply as Hayat or "council", has held frequent talks with Ankara over the years, although Turkey has officially designated them a terrorist organisation. Russia intensified air raids on Azaz and other regions in northern Syria over the past week, underscoring the continuing participation of major powers in the Syrian conflict. “The situation has calmed down since the Turks arrived,” said Rami Al Sayyed, a civil activist in the city. “People also don't want the Hayat to advance further.” Mr Al Sayyed said three civilians were killed in the fighting, and that it was not clear how many militiamen died. Clashes since October 8 have resulted in 10 civilian and 28 rebel fighter deaths, the UK-based Syrian Observatory for Human Rights said. Two years ago, <a href="https://www.thenationalnews.com/mena/2021/10/14/north-west-syria-crops-left-to-rot-as-russia-turkey-tensions-intensify/" target="_blank">Russia and Turkey came close to war</a> after Syrian government forces and its pro-Iran militia allies attacked Turkish proxies in the north. Azaz, situated four kilometres from Turkey, is one of the largest cities still controlled by the opposition, 11 years after the conflict started with street demonstrations against five decades of Assad family rule. The government responded with deadly force and, by the end of 2011, the uprising had become armed. Russian military intervention in 2015 crucially strengthened the government, dominated by the country's Alawite minority. But four million people, mostly Sunni Syrian refugees, still live outside its control in the Turkish sphere of influence in northern Syria, partly carved to contain Kurdish militias, who are separately supported by the US and by Moscow. For years, <a href="https://www.thenationalnews.com/mena/syria/2022/06/19/militant-group-aligned-with-turkey-flexes-muscles-after-fighting-in-northern-syria/" target="_blank">internecine warfare </a>has raged on and off between the Hayat, the most effective anti-Assad group, and other pro-Turkish factions in the north, over turf, and trade and smuggling routes with Turkey, the Kurdish militias, and even with government controlled areas. The Hayat is an amalgamation of several radical groups. Among them is the Al Qaeda-linked Al Nusra Front, which has changed its name several times since it was founded a decade ago. The other anti-Assad factions mostly belong to the Syrian National Army, a de facto Turkish creation. Othman Birakdar, another civil figure in Idlib, said that the Hayat would not have marched into northern Aleppo without the approval of Turkey. “Turkey prefers to deal with one actor rather than with the mess that is the Syrian National Army,” Mr Birakdar said. The Turkish forces deployed near Azaz after the Hayat swept north from its bases in the neighbouring Idlib governorate, overrunning the farming region of Afrin and other areas controlled by the Syrian National Army, which regrouped in Azaz. A Syrian military defector monitoring the situation in the north said that the Syrian National Army has proven inept at governing the north in the five years since it was formed. But Turkey cannot hand all of the area to the Hayat because that could invite Russian retaliation, he said. “It seems Turkey wanted the Hayat to teach the Syrian National Army a lesson but the Hayat went beyond this parameter,” the officer said. The sweep opened a new frontline between the Hayat and pro-Assad forces in the north, as well as between the group and Kurdish militias, known as the People Protection Units (YPG) in the pocket of Tel Rifaat. Six years ago, Russian air raids enabled the YPG to capture Tel Rifaat from the anti-Assad rebels. The US embassy in Syria said on Twitter: “We are alarmed by the recent incursion of Hayat Tahrir Sham, a designated terrorist organisation, into northern Aleppo.” The embassy, which operates outside Syria, said the Hayat's forces “should be withdrawn from the area immediately”.